In French Pat. No. 1,582,543 July 1, 1968, there are described polymerization catalysts for olefins comprising a catalytic solid obtained by reacting a halogenating agent and a derivative of a transition metal with a solid support consisting of an oxygenated compound of a divalent metal. The conditions of the reaction of the solid support with the halogenating agent are selected so as to fix a quantity of halogen so that the atomic ratio of halogen/divalent metal is less than 1. The temperature at which this reaction is carried out is between 20 and 250.degree. C. and preferably between 60 and 150.degree. C. The polymers obtained by means of these catalysts are characterized by a relatively low mean molecular weight and are used principally in applications where molding is carried out by injection. However, they are not entirely suitable for applications where the molding is carried out by extrusion or by blowing.